On my '95, the AC compressor will not cut on at all.
I jumped the pressure switch red to gray wires and the fans cut on but the compressor clutch will not engage.
I didn't have any DTC codes indicating low pressure when I had it on the scanner over the w/e but I'll disconnect the PMC power in a bit just in case.

Anything else to look for?

 

Swapped relays and pulled and checked the fuses and reset the PCM, no luck.
What else can I look for??

 

you can try running jump wires from the ac comp itself to the batt. while running to make the clutch engage. If that works then try isolating the wiring harness for breaks or cuts in the wiring.

BTW are you sure that the a/c systems has enough oil and coolant in the system?

 

No I'm not, it worked 2 months ago fine when we had one of those warm Feb days.
I thought that jumping the pressure switch with the PCM reset would at least make the compressor kick on and cycle if it were low.
There's no obvious signs of leakage as usually you'll see a oily residue were the leak is, like on the front of the compressor, but its all clean as well as any other connection I can see in the tubes.

Good idea on the compressor wire to jump to battery, didn't think of that but was going to hook up the voltmeter to it and see what it does.
I did have the valve covers off a few weeks ago, maybe I pinched that wire going behind the altenator.

 

Today I pulled the relay out to figure out how it works, the AC Compressor relay.
On the bottom of the relay, the pins are labeled.
#87 goes to the Compressor
#85 and #30 have 12V on them.

I jumped pin #87 to #85 and the compressor kicked right on. Let it run a minute or so and no cold air blew so I guess I'm out of freon 134A. But now I need to determine where the leak is....

 

hmmm... okay please excuse my ignorance but what is the issue here? I am having the same symoptoms, compresson wont turn on normally, if i try jumping the wires on the LP port the fan comes on but not the compressor, if i jump the pin 85-87 on the relay the compressor comes on, when i have a pressure gauge hooked up while doing this with the compressor off it is in the red ~150 psi, when the compressor is on the pressure goes into the blue and it blows cold. Why isn't the compresser coming on normally?

 

Well you have the exact same symtoms as I, but my issue turned out to be low pressure, I had less than 20 psi when the compressor was off. Had it charged up with dye, ran for a day or two and can't find any leak anywhere..so I'm in a holding pattern now, AC works fine now that its charged.
Sounds like yours is charged, maybe the pressure switch isn't working properly...and BTW, I have no idea why the compressor is not kicking on when jumping the switch, there must be something else involved there but mine just made the fans come on like yours, but no compressor, even with the PCM reset.

 

so reseting the pcm might fix it? i'll try that

 

Supposedly when low pressure is detected it sets a code in the PCM, when this code is set, the compressor will not turn on even if the switch is jumped.
I reset mine and I still could not get the compressor to kick on and I didn't have any codes stored before hand either according to my data master....so I'm not sure how it is exactly supposed to work. It would make sense that the PCM would keep it from kicking on in order to prevent premature compressor failure when there was not enough pressure at some point for some reason to work properly. I pulled both the fuse in the dash and the one in the fuse /relay box under the hood to reset it.

 

tried resetting the pcm, no luck, any other ideas, definatly seems to be electrical since jumping the relay makes it work, but jumping the lp sensor doesnt work? any thoughts? ways to check the lp sensor with a multi meter?

 

I have no idea, I'm sure there is a way to check it with a multimeter.
I'd say now that mine works I would jump the pressure switch and see what mine does, but I started tearing it down today for a head swap.
I'll check in the morning and see if my '99 has the same switch and see how it works... Otherwise, you may want to post a new question on it, but mine didn't get much help from anyone.

 

Okay thanks everyone, problem fixed, basically when i re-charged the ac system the one can i used was not enough, after putting in just the one can the gauge read in the "filled" range, at idle. However when the rpms raised the pressure dropped down barely to the "low" range which tripped the lp sensor like we all thought. The thing that was throwing it all off was that the PCM needs to be reset to bring the compressor back online. So after resetting the pcm and watching the needle while having someone run up the rpms we added another lb and now it is all good... for now